An alarming number of adults and children in the United States are either overweight or obese. Healthier food choices, including foods that are high in resistant starch, can help people to better manage their blood sugar levels and their weight. Resistant starch is defined as starch that is not digested in the small intestine of healthy individuals but is fermented in the large intestine. Due to its slow digestion, resistant starch does not have the same caloric load as readily digestible starch, nor does it cause as rapid a rise in blood glucose levels after ingestion. Instead, resistant starch results in a more controlled glucose release over a longer period of time after digestion. This results in a decreased glycemic response, increased insulin sensitivity, and greater feelings of satiety. As a form of dietary fiber, resistant starch contributes to better colon health due to its fermentation by probiotic organisms in the lower gastrointestinal tract into short chain fatty acids, such as butyrate.
In the United States, the majority of dietary starch is consumed in the form of wheat based foods, such as bread, cereals, pastas, and tortillas, which contain very low levels of resistant starch. Cereal starches typically contain less slowly digested amylose (about 25% of total starch) and more highly branched, rapidly digested amylopectin (about 75% of total starch). The amount of amylose in starch positively correlates with the levels of dietary fiber and resistant starch. In corn and barley, loss-of-function mutations of SBEIIb, one of several enzymes in the starch synthesis pathway, have been identified. SBEIIb is the predominant isoform of SBEII expressed in the endosperm of these crops and its loss results in increased amylose and resistant starch levels. In contrast, both SBEIIa and SBEIIb are expressed in the wheat endosperm, but SBEIIa is the major isoform that is expressed in this crop. Though there has been great interest in finding mutations that increase amylose content (and therefore resistant starch content) in wheat, wheat lines with increased amylose levels are not commercially available. Preferred mutations would be single nucleotide polymorphisms (SNPs) that reduce or eliminate SBEII enzyme activity (and, in turn, increase amylose levels) without having significant negative pleiotropic effects.
Identification of SNPs in wheat SBEII genes has proceeded slowly because, among other possible reasons, there is limited genetic diversity in today's commercial wheat cultivars and bread wheat is a polyploid, with a complement of 7 chromosomes from each of three ancestors called the A, B and D genomes, resulting in a total of 21 chromosomes. Typically, the bread wheat genome has three functionally redundant copies of each gene (called homoeologs), and therefore, single gene alterations usually do not produce any readily visible phenotype such as those that have been found in diploid corn. Often in wheat, altered variants of all three homoeologs must be combined genetically in order to evaluate their effects. Pasta (durum) wheat is a tetraploid, consisting of A and B genomes, so only two altered copies of each homoeolog must be combined to obtain a phenotype.
To further compound these challenges, SBEIIa and SBEIIb are closely located on the same chromosome in wheat, making it difficult for alleles in these genes to be inherited independently unless through a rare recombination event. Thus, it would be useful to have knock-down or knock-out mutations, resulting from SNPs, of both SBEIIa and SBEIIb of each genome of wheat. The availability of multiple allelic mutations within each SBEII locus, particularly within each SBEII locus of the same genome, would allow for the breeding of new, non-genetically modified wheat lines with a spectrum of increased amylose and resistant starch levels in seeds. Seeds from these lines could be used to produce healthier wheat-based food products, including flour, bread, cereals, pastas, and tortillas.